The goal of this project is examine the kinetics and mechanism of NADH photobleaching under different irradiation conditions. Much of our research centers around autofluorescence imaging of living cells and tissue. Since NADH is regulated by the cell and important for proper cell function, bleaching during imaging could have a profound effect on the health of the cell. In the preliminary experiments we measured the one-photon bleaching cross section with low intensity irradiation. Future measurements will examine and compare one and two photon high intensity illumination. Since the primary photobleaching mechanism cited in the literature for one photon excitation involves the generation of enzymatically active NAD, we will measure and compare the fraction of bleached NADH that leads to enzymatically active NAD for the different irradiation conditions..